Young’s Double Slit
Contents:
•Interference
•Diffraction
•Young’s Double Slit
•Angle
•Distance
•Single slit
•Rayleigh Criterion
Interference
Sources
L1
L2
When the difference in distance (L1- L2) is an
integer number of wavelengths (0, 1, 2, 3…) = Constructive
remainder of a half (1/2, 1 1/2, 2 1/2, …) = Destructive
TOC
Young’s Double Slit Experiment
Monochromatic, coherent light
Light spreads out from slits
Screen has sum of two sources
Interference pattern on screen
Demo
TOC
Young’s Double Slit Experiment
L1
d
Next Maxima
Central Maxima
L2
L2 – L1 = d sin
d = slit separation
TOC
Young’s Double Slit Experiment
L1
d
Next Maxima

Central Maxima
L2
L2 - L2 = d sin
d = slit separation
d sin = n (n = 0, 1, 2…) - constructive
d sin = (n +1/2) - destructive
TOC
Young’s Double Slit - Angle
1|2|3|4
TOC
Two slits are 3.00 x 10-6 m apart. What is the
wavelength of the incident light if the bright
fringes are separated by 6.00o?
d sin = n
d = 3.00 x 10-6 m, n = 1,  = 6.00o
314 nm
W
Two slits are 9.00 x 10-6 m apart. If 565 nm light
is incident, what angle separates the central
maxima, and the third fringe out?
d sin = n
d = 9.00 x 10-6 m, n = 3,  = 565 x 10-9 m
10.9o
W
Young’s Double Slit Experiment
L1
d
Next Maxima

D
s
Central Maxima
Similar triangles - set up proportion
s= 
D d
TOC
Young’s Double Slit Distance
1|2|3|4
TOC
520 nm light hits two slits separated by 2.5 x 10-5
m. What distance separates the bright fringes on
the screen that is 5.0 m distant?
s =
D

d
d = 2.5 x 10-5 m, D = 5.0 m,  = 520 x 10-9 m
.10 m
W
Light hits two slits separated by 2.50 x 10-5 m. If
5.70 cm separates the bright fringes on the screen
that is 5.00 m distant, what is the wavelength of
light being used?
s =
D

d
d = 2.50 x 10-5 m, D = 5.00 m, d = .0570 m
285 nm
W
A single slit can make an
interference pattern too
Rayleigh Criterion
 = 1.22
b
 = Angle of resolution (Rad)
 = Wavelength (m)
b = Diameter of circular opening (m)
(Telescope aperture)
the bigger the aperture, the smaller the
angle you can resolve.
Central maximum of one is over
minimum of the other
Rayleigh Criterion
1|2
TOC
What is the angular resolution of the 100 inch
(2.54 m) diameter telescope on the top of Mt
Wilson? (use 550 nm as the wavelength)
(uh 550 nm = 550 x 10-9 m)
 = 1.22
b
 = ?,  = 550 x 10-9 m, b = 2.54 m
 = 2.64173E-07
2.6 x 10-7 radians
W
What diameter telescope do you need to resolve
two stars that are separated by 1.8 x 1011 m, but
are 3.0 × 1017 m from us? (use 550 nm as the
wavelength)
(AU, 32 LY)
hint  = s/r = (1.8 x 1011 m)/(3.0 × 1017 m)
 = 1.22
b
 = 6.00 x 10-7,  = 550 x 10-9 m, b = ?
b = 1.12 m
1.1 m
W